Flexographic printing precursors and methods of making

ABSTRACT

A mixture of a high molecular weight EPDM rubber with a low molecular weight (liquid) EPDM rubber provides a highly useful laser-ablatable flexographic printing plate precursor formulation. This formulation is sensitive to infrared radiation by the incorporation of an IR absorbing compound such as a carbon black. The inclusion of the liquid EPDM rubber avoids the need for plasticizers such as process oils during manufacturing, and provides improved image sensitivity, print quality, and run length. Both flexographic printing plates and printing sleeves can be made using the mixture of EPDM rubbers.

FIELD OF THE INVENTION

This invention relates to flexographic printing precursors in the formof either plates or sleeves that contain an IR-ablatable relief-forminglayer comprising a mixture of rubbery resins. This invention alsorelates to a method of preparing these flexographic printing precursorsin either plate or sleeve form.

BACKGROUND OF THE INVENTION

Flexography is a method of printing that is commonly used forhigh-volume printing runs. It is usually employed for printing on avariety of substances particularly those that are soft and easilydeformed, such as paper, paperboard stock, corrugated board, polymericfilms, fabrics, plastic films, metal foils, and laminates. Coursesurfaces and stretchable polymeric films can be economically printed bythe means of flexography.

Flexographic printing plates are sometimes known as “relief printingplates” and are provided with raised relief images onto which ink isapplied for application to the printing substance. The raised reliefimages are inked in contrast to the relief “floor” that remains free ofink in the desired printing situations. Such printing plates aregenerally supplied to the user as a multi-layered article having one ormore imageable layers coated on a backing or substrate. Flexographicprinting can also be carried out using a flexographic printing cylinderor seamless sleeve having the desired raised relief image. Theseflexographic printing cylinder or sleeve precursors can be “imagedin-the-round” (ITR), either by using a standard photomask or a “laserablation mask” (LAM) imaging on a photosensitive plate formulation, orby “direct laser engraving” (DLE) of a plate precursor that is notnecessarily photosensitive.

U.S. Pat. No. 5,719,009 (Fan) describes elements having an ablatablelayer disposed over photosensitive layer(s) so that after imageablation, UV exposure of the underlying layer hardens it whilenon-exposed layer(s) and the ablatable mask layer are subsequentlywashed away.

DuPont's Cyrel® FAST™ thermal mass transfer plates are commerciallyavailable photosensitive resin plate precursors that comprise anintegrated ablatable mask element and require minimal chemicalprocessing, but they do require thermal wicking or wiping to remove thenon-exposed areas. These also require extensive disposal of polymericwaste and some drying of the processed (developed) plates.

There remains a need for a totally processless method of producingflexographic printing plates with high throughput efficiency. A methodfor forming a relief pattern on a printing element by directly engraving(DE) with a laser is already used to produce relief plates and stamps.However, the requirement of relief depths in excess of 500 μm challengesthe speed at which these flexographic printing plate precursors can beimaged. In contrast to the laser ablation of the CTP mask layers atopthe photosensitive resin, which only requires low energy lasers and lowfluence, the DE of laser ablatable flexographic printing plates requireshigher energy lasers and higher fluence. In addition, the laserablatable, relief-forming layer becomes the printing surface and musthave the appropriate physical and chemical properties needed for goodprinting. The laser engravable black mask layer is washed away duringthe development and is not used during the printing.

Flexographic printing plate precursors used for infrared radiation (IR)laser ablation engraving must comprise an elastomeric or polymericcomposition that includes one or more infrared radiation absorbingcompounds. When the term “imaging” is used in connection with “laserengraving”, it refers to ablation of the background areas while leavingintact the areas of the element that will be inked and printed in aflexographic printing station or press.

Commercial laser engraving is typically carried out using carbon dioxidelasers. While they are generally slow and expensive to use and have poorbeam resolution, they are used because of the attractions of directthermal imaging. Infrared (IR) fiber lasers are also used. These lasersprovide better beam resolution, but are very expensive. IR laserengravable flexographic printing plate blanks having unique engravablecompositions are described in WO 2005/084959 (Figov).

Direct laser engraving is described, for example, in U.S. Pat. Nos.5,798,202 and 5,804,353 (both Cushner et al.) in which various means areused to reinforce the elastomeric layers. The reinforcement can be doneby addition of particulates, by photochemical reinforcement, or bythermochemical hardening. U.S. Pat. No. 5,804,353 describes a multilayerflexographic printing plate wherein the composition of the top layer isdifferent from the composition of the intermediate layer. Carbon blackcan be used as a reinforcing agent and can be present in both layers.

Flexographic printing plate precursors for near-IR laser ablationengraving generally comprise an elastomeric or polymeric system that ismade thermosetting by a polymerization reaction and includes fillers andinfrared absorbing compounds. During recent years, infrared laser diodeshave been used for ablation of thin layers (U.S. Pat. No. 5,339,737 ofLewis et al.) for use in offset lithographic printing. These lasers arebecoming increasingly inexpensive and more powerful and consequently arebecoming more useful for laser ablation of thick layers such as arefound in flexographic printing precursors. Such lasers require thepresence of radiation absorbing dyes or pigments in the flexographicprinting precursors as they generally operate around wavelengths of 800nm to 1200 nm. They have the potential to enable faster imaging, higherprint quality, and more reliable engraving than obtained with carbondioxide lasers. In addition, it is advantageous to optimize imagingspeed by formulating printing plates with higher sensitivity. This willgive higher productivity in printing plate production with the potentialof greater profits for the printing houses or trade shops where theprinting plates may be produced. Imaging systems can be made by usingarrays of laser diodes. Throughput also depends on the number of laserdiodes being used and there is a balance between the cost of imagingheads that depends on the number of diodes and their combined outputpower. The requirement for high print quality has increased considerablyin recent years as flexographic printing penetrates markets previouslydominated by high quality offset lithography. Laser engraving usinginfrared diodes instead of carbon dioxide provides an opportunity forhigher quality because the wavelength of the diode radiation at 800-1000nm is so much smaller than that of carbon dioxide of 10.7 μm.

As mentioned above, the chosen commercial means of imaging by laserengraving has for some years been with carbon dioxide lasers. These arecapable of ablating layers to produce suitable relief depths forflexographic printing. Such depths may be anywhere in the range from 200μm to 5 mm. As carbon dioxide lasers operate at a wavelength of 10.7 μm,there is no need to incorporate infrared absorbing dyes or pigments intothe printing precursors because the polymers themselves absorb at thiswavelength for ablation.

Although patents concerned with formulating laser-engraved printingprecursors may mention laser diode engraving, they have been primarilyaimed at carbon dioxide laser imaging and thus include formulationslacking infrared absorbing materials as described in U.S. Pat. No.5,259,311 (MacCaughey). Formulations designed for ablation by carbondioxide lasers cannot be easily modified for laser diode ablation bysimply adding a suitable infrared radiation absorbing material. Forinstance, infrared dyes may react with the chemistry used to vulcanizethe last-ablatable layer, or carbon black may block UV radiation usedfor curing the flexographic precursor composition.

One approach to formulation of laser-engravable flexographic printingprecursors is to produce thermoplastic formulations that have not beencrosslinked to form thermoset materials. These have been found to be oflimited suitability for laser engraving because ablation ofthermoplastic materials results in melted portions around the ablatedareas and sometimes re-deposition of ablated material onto the ablatedareas. This is because it is inevitable that during imaging there isheat flowing to non-imaged areas that is insufficient for ablation butsufficient for melting, as described in U.S. Patent ApplicationPublication 2004/0231540 (Hiller et al.).

A number of elastomeric systems have been considered for construction oflaser-engravable flexographic printing precursors. The earliestformulations included natural rubbers (as reported in U.S. Pat. No.6,223,655 by Shanbaum et al. using a mixture of epoxidized naturalrubber and natural rubber). Also, engraving of a rubber is described in“Laser Material Processing of Polymers” by S. E. Nielsen in PolymerTesting 3 (1983) 303-310.

U.S. Pat. No. 4,934,267 (Hashimito) describes the use of natural rubberor synthetic rubber or mixtures of both and specifically mentionsacrylonitrile-butadiene, styrene-butadiene and chloroprene with atextile support. “Laser Engraving of Rubbers—The influence of Fillers”by W. Kern et al., October 1997, pp. 710-715 (Rohstoffe UndAnwendendunghen) described the use of natural rubber, nitrile rubber(NBR), ethylene-propylene-diene terpolymer (EPDM), and styrene-butadienecopolymer (SBR). The article entitled “Laser Engraving of Rubbers—TheUse of Microporous Materials” by Kern et al., 1998 described the use ofnatural rubber compounds and EPDM.

EP1,228,864B1 (Houstra) describes liquid photopolymer mixtures designedfor analogue UV imaging, cured with UV, and then the resulting platesare laser engraved using carbon dioxide. Such printing plate precursorsdo not contain infrared absorbing dyes or pigments and therefore areunsuitable for use with IR absorbing laser diode systems. U.S. Pat. No.5,798,202 (noted above) describes reinforced block copolymersincorporating carbon black that is UV cured and is still thermoplastic.As pointed out in U.S. Pat. No. 6,935,236 (Hiller et al.), such curingwould be defective due to the high absorption of UV as it traversesthrough the thick precursor layer. The block copolymers described inCushner et al. are the basis of most commercial UV-imageableflexographic printing precursors. Although many polymers are suggestedfor this use in the prior art, only polymers that are extremely flexiblesuch as elastomers have been used commercially. This is becauseflexographic layers that are millimeters thick need to be bent around aprinting cylinder and secured with temporary bonding tape that must bothbe removable after printing and secure the printing plate duringprinting.

U.S. Pat. No. 6,776,095 (Telser et al.) lists a number of elastomersincluding EPDM and U.S. Pat. No. 6,913,869 (Leinenbach et al.) describesthe use of EPDM rubber for the production of flexographic printingplates having a flexible metal support. U.S. Pat. No. 7,223,524 (Hilleret al.) describes the use of natural rubber with highly conductivecarbon blacks with specific properties of structure and surface area.U.S. Pat. No. 7,290,487 (Hiller et al.) lists suitable hydrophobicelastomers with inert plasticizers. U.S. Patent Application Publication2002/0018958 (Nishioki et al.) describes a peelable layer and the use ofrubbers such as EPDM and NBR together with inert plasticizers such asmineral oils. The use of inert plasticizers or mineral oils can presenta problem as they leach out either during precursor grinding (duringmanufacture) or during storage, or under the pressure and contact withink during printing.

An increased need for higher quality flexographic printing precursorsfor laser engraving has highlighted the need to increase the desire tosolve performance problems that may have been of less importance whenquality demands were less. What is especially difficult is tosimultaneously improve the flexographic printing precursor in alldirections.

For example, the rate of imaging is now an important consideration inlaser engraving of flexographic printing precursors. Throughput byengraving depends upon printing plate width because it is imaged pointby point. Conventional printing plates made by UV exposure followed bymultiprocessing wash-out and drying is time consuming but is independentof printing plate size, and for the production of multiple printingplate, it can be relatively fast because many printing plates can bepassing through the multiple stages at the same time. Throughput forflexographic engraving is somewhat determined by the equipment that isused but if this is the means for improving imaging speed, then costbecomes the main factor. Improved imaging speed is related to equipmentcost. There is a limit to what the market will bear in equipment cost inorder to have faster imaging. Therefore, much work has been done to tryto improve the sensitivity of the flexographic printing plate by variousmeans. For instance, U.S. Pat. No. 6,159,659 (Gelbart) describes the useof a foam layer for laser engraving so that there is less material toablate. U.S. Pat. No. 6,806,018 (Kanga) uses expandable microspheres toincrease sensitivity.

U.S. Patent Application Publication 2009/0214983 (Figov et al.)describes the use of additives that thermally degrade to produce gaseousproducts. U.S. Patent Application Publication 2008/0194762 (Sugasaki)suggests that good imaging sensitivity can be achieved using a polymerwith a nitrogen atom-containing hetero ring. U.S. Patent ApplicationPublication 2008/0258344 (Regan et al.) describes laser-ablatableflexographic printing precursors that can be degraded to simplemolecules that are easily removed.

There continues to be a need to provide improved flexographic printingprecursors that are easily manufactured without the use of process oilsthat have improved sensitivity (imaging speed) and provide improvedprint quality and run length.

SUMMARY OF THE INVENTION

The present invention includes an infrared radiation ablatableflexographic printing precursor that comprises an infrared radiationablatable layer comprising a mixture of a high molecular weightethylene-propylene-diene terpolymer (EPDM) rubber and a low molecularweight EPDM rubber.

In some embodiments, an infrared radiation ablatable flexographicprinting precursor comprises an infrared radiation ablatable layercomprising from about 1 to about 20 weight % of a conductive carbonblack having a dibutyl phthalate (DBP) adsorption of less than 110, anda mixture a high molecular weight ethylene-propylene-diene terpolymer(EPDM) rubber and a low molecular weight EPDM rubber, wherein the weightratio of the high molecular weight EPDM to the low molecular weight EPDMrubber is from about 3:1 to about 5:1.

In still other embodiments, an infrared radiation ablatable flexographicprinting precursor comprises an infrared radiation ablatable layercomprising one or more inorganic fillers, an infrared radiationabsorbing material (such as a carbon black), and a mixture a highmolecular weight ethylene-propylene-diene terpolymer (EPDM) rubber and alow molecular weight EPDM rubber, wherein the weight ratio of the highmolecular weight EPDM to the low molecular weight EPDM rubber is fromabout 2:1 to about 10:1.

Other embodiments of this invention includes an infrared radiationablatable flexographic printing precursor comprises an infraredradiation ablatable layer comprising a carbon black, one or moreinorganic fillers, and a mixture of a high molecular weightethylene-propylene-diene terpolymer (EPDM) rubber and a low molecularweight EPDM rubber, wherein the weight ratio of the carbon black to theinorganic filler(s) is from about 1:50 to about 1:1.5.

Still again, other embodiments of this invention include an infraredradiation ablatable flexographic printing precursor comprises aninfrared radiation ablatable layer comprising:

from about 10 to about 35 weight % of one or more inorganic fillers andfrom about 1 to about 20 weight % of a carbon black, wherein the weightratio of the carbon black to the inorganic filler(s) is from about 1:50to about 1:1.5, and

a mixture of a high molecular weight ethylene-propylene-diene terpolymer(EPDM) rubber and a low molecular weight EPDM rubber, wherein the weightratio of the high molecular weight EPDM to the low molecular weight EPDMrubber is from about 2:1 to about 10:1.

This invention also provides a method of preparing the flexographicprinting plate precursor of this invention comprising:

A) providing a mixture of a high molecular weightethylene-propylene-diene terpolymer (EPDM) rubber and a low molecularweight EPDM rubber,

B) adding optional components, and compounding the resulting mixture ina two-roll mill,

C) applying the compounded mixture to a fabric base to provide acontinuous roll of an infrared radiation ablatable layer,

D) causing vulcanization in the continuous infrared radiation ablatablelayer,

E) laminating a polyester support to the continuous infrared radiationablatable layer to provide a continuous laminated web, and

F) grounding the infrared radiation ablatable layer.

In addition, a method of this invention for preparing the flexographicprinting sleeve precursor of this invention comprises:

A) providing a mixture of a high molecular weightethylene-propylene-diene terpolymer (EPDM) rubber and a low molecularweight EPDM rubber,

B) adding optional components, and compounding the resulting mixture ina two-roll mill,

C) applying the compounded mixture to a printing sleeve core to providean infrared radiation ablatable sleeve,

D) causing vulcanization in the infrared radiation ablatable sleeve, and

E) smoothing the continuous infrared radiation ablatable sleeve to auniform thickness.

A method of this invention for providing flexographic printing plate orsleeve comprising:

imaging the flexographic printing precursor of this invention usinginfrared radiation to provide a relief image in the infrared radiationablatable layer.

In addition, this invention provides a system for providing aflexographic printing plate or printing sleeve, comprising:

the flexographic printing precursor of this invention,

a group of one or more sources of imaging infrared radiation, eachsource capable of emitting infrared radiation,

a set of optical elements coupled to the sources of imaging infraredradiation to direct imaging infrared radiation from the sources onto theflexographic printing precursor.

The present invention provides a laser engravable flexographic printingprecursor that is readily manufactured without using process oils, andhaving improved image sensitivity, print quality, and run length.

Whereas prior art researchers have used high molecular weight EPDMrubber as well as other rubbery materials, they have failed toappreciate that its non-polar nature has made it particularly suitableas a basis for laser-engravable flexographic printing precursors andsuperior to other materials that are described in patents. Furthermore,we have found advantages from the inclusion of a low molecular weightEPDM polymer with the high molecular weight EPDM rubber, as areplacement for plasticizers. Low molecular weight EPDM provides thebenefits of process oils in manufacture without the problems of leachingout either during grinding, printing or precursor storage.

We have also found that the use of the low molecular weight EPDM polymercauses an increase in crosslinking density in the rubber mixture withconsequent advantages. For example, there is an improvement incompression set and mechanical properties such as tensile strength andelongation to the length at which the material breaks or snaps into atleast two pieces (see ASTM D3759).

The present invention provides improved flexographic printing precursorsthat can be in the form of plates or sleeves. These precursors can becleanly engraved using infrared radiation (lasers) to provide very sharpfeatures in the resulting printed images. In addition, these precursorshave improved run length and can be used for many high quality printswithout degradation. These advantages are also provided by using aspecific mixture of solid (high molecular weight) EPDM and liquid (lowmolecular weight) EPDM to formulate the infrared radiation ablatablelayers.

DETAILED DESCRIPTION OF THE INVENTION

“Imaging” refers to ablation of the background areas while leavingintact the areas of the plate precursor that will be inked up andprinted by flexography.

“Flexographic printing precursor” refers to a non-imaged flexographicelement.

The terms “laser-ablatable element”, “flexographic printing precursor”,“flexographic printing plate precursor”, and “flexographic printingsleeve precursor” used herein includes any imageable element or materialof any form in which a relief image can be produced using a laseraccording to the present invention. In most instances, however, thelaser-ablatable elements are used to form flexographic printing plates(flat sheets) or flexographic printing sleeves with a relief imagehaving a relief depth of at least 100 μm. Such laser-ablatable,relief-forming elements may also be known as “flexographic printingplate blanks” or “flexographic sleeve blanks” The laser-ablatableelements can also be in seamless continuous forms.

By “ablative”, we mean that the imageable (or infrared radiationablatable) layer can be imaged using an infrared radiation source (suchas a laser) that produces heat within the layer that causes rapid localchanges in the infrared radiation-ablatable layer so that the imagedregions are physically detached from the rest of the layer or substrateand ejected from the layer and collected by a vacuum system. Non-imagedregions of the infrared radiation-ablatable layer are not removed orvolatilized to an appreciable extent and thus form the upper surface ofthe relief image that is the printing surface. The breakdown is aviolent process that includes eruptions, explosions, tearing,decomposition, fragmentation, oxidation, or other destructive processesthat create a broad collection of materials. This is distinguishablefrom, for example, image transfer. “Ablation imaging” is also known as“ablation engraving” in this art. It is distinguishable from imagetransfer methods in which ablation is used to materially transfer animage by transferring pigments, colorants, or other image-formingcomponents.

Unless otherwise indicated, the term “weight %” refers to the amount ofa component or material based on the total dry layer weight of thecomposition or layer in which it is located.

The “top surface” is equivalent to the “relief-image forming surface”and is defined as the outermost surface of the infraredradiation-ablatable layer and is the first surface of that layer that isstruck by imaging infrared radiation during the engraving or imagingprocess. The “bottom surface” is defined as the surface of the infraredradiation-ablatable layer that is most distant from the imaging infraredradiation.

Flexographic Printing Precursor

The flexographic printing precursors can include a self-supportinginfrared radiation ablatable layer (defined below) that does not need aseparate substrate to have physical integrity and strength. In suchembodiments, this layer is thick enough and laser ablation is controlledin such a manner that the relief image depth is less than the entirethickness, for example at least 20% but less than 80% of the entirethickness.

However, in other embodiments, the flexographic printing precursor has asuitable dimensionally stable, non-laser ablatable substrate having animaging side and a non-imaging side. The substrate has at least oneinfrared radiation ablatable layer disposed on the imaging side.Suitable substrates include dimensionally stable polymeric films,aluminum sheets or cylinders, transparent foams, ceramics, fabrics, orlaminates of polymeric films (from condensation or addition polymers)and metal sheets such as a laminate of a polyester and aluminum sheet orpolyester/polyamide laminates, or a laminate of a polyester film and acompliant or adhesive support. Polyester, polycarbonate, polyvinyl, andpolystyrene films are typically used. Useful polyesters include but arenot limited to poly(ethylene terephthalate) and poly(ethylenenaphthalate). The substrates can have any suitable thickness, butgenerally they are at least 0.01 mm or from about 0.05 to about 0.3 mmthick, especially for the polymeric substrates. An adhesive layer may beused to secure the laser-ablatable layer to the substrate.

There may be a non-laser ablatable backcoat on the non-imaging side ofthe substrate (if present) that may be composed of a soft rubber orfoam, or other compliant layer. This backcoat may be present to provideadhesion between the substrate and the printing press rollers and toprovide extra compliance to the resulting printing plate, or to reduceor control the curl of the printing plate.

The flexographic printing precursor contains one or more layers. Thatis, it can contain multiple layers, at least one of which is an infraredradiation ablatable layer in which the relief image is formed. Forexample, there may be a non-laser ablatable elastomeric rubber layer(for example, a cushioning layer) between the substrate and the infraredradiation ablatable layer.

In most embodiments, the infrared radiation ablatable layer is theoutermost layer, including embodiments where that layer is disposed on aprinting cylinder as a sleeve. However, in some embodiments, theinfrared radiation ablatable layer can be located underneath anoutermost capping smoothing layer that provides additional smoothness orbetter ink reception and release. This smoothing layer can have ageneral thickness of from about 1 to about 200 μm.

In general, the infrared radiation ablatable layer has a thickness of atleast 50 μm and generally from about 50 to about 4,000 μm, and typicallyfrom 200 to 2,000 μm.

The infrared radiation ablatable layer and formulation includes one ormore high molecular weight ethylene-propylene-diene terpolymer (EPDM)rubbers. These rubbers generally have a molecular weight of from about200,000 to about 800,000 and more typically from about 250,000 to about500,000, or optimally, about 300,000. The high molecular weight rubbersare generally solid form and the molecular weight is at least 30 times(or even 50 times) higher than that of the low molecular weight EPDMrubbers. The high molecular weight EPDM rubbers can be obtained from anumber of commercial sources as the following products: Keltan® EPDM(from DSM Elastomers) and Royalene® EPDM (from Lion Copolymers).

In addition, this layer includes one or more low molecular weight EPDMrubbers that are usually in liquid form, and having a molecular weightof from about 2,000 to about 10,000 and typically from about 2,000 toabout 8,000. These components are also available from various commercialsources, for example as Trilene® EPDM (from Lion Copolymers).

These two essential components are present at a weight ratio (highmolecular weight EPDM rubber to low molecular weight EPDM) of from about2:1 to about 10:1, or from about 3:1 to about 5:1. Higher ratios do notaffect the tack of the mixture sufficiently to permit good calenderingand lower ratios give formulations that become too tacky and areconsequently hard to handle, resulting in flexographic printingprecursors that are too brittle for practical use.

The amount of the high molecular weight EPDM in the infrared radiationablatable layer is generally at least 15 and up to and including 70weight %, based on the total dry layer weight. More typically, theamount is from about 25 to about 45 weight %. Thus, all components otherthan the two EPDM rubbers are present in an amount of no more than 80weight %, or typically no more than 60 weight %, based on the total drylayer weight.

The infrared radiation ablatable layer may also include minor amounts(less than 40 weight % of the total polymers or resins in the layer) ofother “secondary” resins that are often included in laser-ablatablelayers. These materials may need the presence of an intermediatebridging material to maintain compatibility. Such resins can include butare not limited to, crosslinked elastomeric or rubbery resins that arefilm-forming in nature. For example, the elastomeric resins can bethermosetting or thermoplastic urethane resins and derived from thereaction of a polyol (such as polymeric diol or triol) with apolyisocyanate, or the reaction of a polyamine with a polyisocyanate.Alternatively, such polymers consist of a thermoplastic elastomer and athermally initiated reaction product of a multifunctional monomer oroligomer.

Other elastomeric resins include copolymers of styrene and butadiene,copolymers of isoprene and styrene, styrene-butadiene-styrene blockcopolymers, styrene-isoprene-styrene copolymers, other polybutadiene orpolyisoprene elastomers, nitrile elastomers, polychloroprene,polyisobutylene and other butyl elastomers, any elastomers containingchlorosulfonated polyethylene, polysulfide, polyalkylene oxides, orpolyphosphazenes, elastomeric polymers of (meth)acrylates, elastomericpolyesters, and other similar polymers known in the art.

Still other useful secondary resins include vulcanized rubbers, such asNitrile (Buna-N), Natural rubber, Neoprene or chloroprene rubber,silicone rubber, fluorocarbon rubber, fluorosilicone rubber, SBR(styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber),ethylene-propylene rubber, and butyl rubber.

Still other useful secondary polymers include but are not limited to,poly(cyanoacrylate)s that include recurring units derived from at leastone alkyl-2-cyanoacrylate monomer and that forms such monomer as thepredominant low molecular weight product during ablation. These polymerscan be homopolymers of a single cyanoacrylate monomer or copolymersderived from one or more different cyanoacrylate monomers, andoptionally other ethylenically unsaturated polymerizable monomers suchas (meth)acrylate, (meth)acrylamides, vinyl ethers, butadienes,(meth)acrylic acid, vinyl pyridine, vinyl phosphonic acid, vinylsulfonic acid, and styrene and styrene derivatives (such asα-methylstyrene), as long as the non-cyanoacrylate comonomers do notinhibit the ablation process. The monomers used to provide thesepolymers can be alkyl cyanoacrylates, alkoxy cyanoacrylates, andalkoxyalkyl cyanoacrylates. Representative examples ofpoly(cyanoacrylates) include but are not limited to poly(alkylcyanoacrylates) and poly(alkoxyalkyl cyanoacrylates) such aspoly(methyl-2-cyanoacrylate), poly(ethyl-2-cyanoacrylate),poly(methoxyethyl-2-cyanoacrylate), poly(ethoxyethyl-2-cyanoacylate),poly(methyl-2-cyanoacrylate-co-ethyl-2-cyanoacrylate), and otherpolymers described in U.S. Pat. No. 5,998,088 (Robello et al.)

In other embodiments, the secondary polymers are an alkyl-substitutedpolycarbonate or polycarbonate block copolymer that forms a cyclicalkylene carbonate as the predominant low molecular weight productduring depolymerization from ablation. The polycarbonate can beamorphous or crystalline as described for example in U.S. Pat. No.5,156,938 (Foley et al.), Cols. 9-12.

The infrared radiation ablatable layer can also include one or moreinfrared (IR) radiation absorbing compounds that absorb IR radiation inthe range of from about 750 to about 1400 nm or typically from 800 to1250 nm as long as they do not interfere with the vulcanization process.Particularly useful infrared radiation absorbing compounds areresponsive to exposure from IR lasers. Mixtures of the same or differenttypes of infrared radiation absorbing compounds can be used if desired.

A wide range of infrared radiation absorbing compounds are useful in thepresent invention, including carbon blacks and other IR radiationabsorbing organic or inorganic pigments (including squarylium, cyanine,merocyanine, indolizine, pyrylium, metal phthalocyanines, and metaldithiolene pigments), and metal oxides. Examples include RAVEN® 450,RAVEN® 760 ULTRA®, RAVEN® 890, RAVEN® 1020, RAVEN® 1250 and others thatare available from Columbian Chemicals Co. (Atlanta, Ga.) as well as N330 and N 772 that are available from Evonik Industries AG(Switzerland). Carbon blacks and especially conductive carbon blacks(described below) are particularly useful.

Useful IR radiation absorbing compounds also include carbon blacks thatare surface-functionalized with solubilizing groups are well known inthe art. Carbon blacks that are grafted to hydrophilic, nonionicpolymers, such as FX-GE-003 (manufactured by Nippon Shokubai) are alsouseful. Other useful carbon blacks are Mogul L, Mogul E, Emperor 2000,and Regal® 330, and 400, all from Cabot Corporation (Boston Mass.).Other useful pigments include, but are not limited to, Heliogen Green,Nigrosine Base, iron (III) oxides, transparent iron oxides, magneticpigments, manganese oxide, Prussian Blue, and Paris Blue. Other usefulIR radiation absorbing compounds are carbon nanotubes, such as single-and multi-walled carbon nanotubes, graphite, grapheme, and porousgraphite.

Conductive carbon blacks can also be used in the practice of thisinvention. Such carbon blacks can be acidic or basic in nature and canhave a dibutyl phthalate (DBP) absorption value less than 110 (110ml/100 g), as opposed to conductive carbon blacks having high DBPabsorption values that are generally known for commercial conductivecarbon blacks. Lower DBP absorption values are desirable because theyprovide lower viscosity of the infrared radiation ablatable layerformulations, making easier the manufacture of the flexographic printingprecursors. Useful conductive carbon blacks can be obtained commerciallyas Ensaco™ 150 P (from Timcal Graphite and Carbon), Hi Black 160 B (fromKorean Carbon Black Co. Ltd.), and N 293 (from Evonik Industries).

Electrically conductive carbon blacks with low DBP absorptions (asmeasured using ASTM D2414-82 DBP Absorption of Carbon Blacks) or low BETsurface area (BET nitrogen surface area as measured by ASTM D 3037-89)are preferred. High DBP absorption or high surface area blacks produceformulations with too high a viscosity and cause handling problemsduring manufacture.

A finer dispersion of very small particles of pigmented IR radiationabsorbing compounds can provide an optimum ablation feature resolutionand ablation efficiency. Particularly suitable particles are those withdiameters less than 1 μm.

Dispersants and surface functional ligands can be used to improve thequality of the carbon black or metal oxide, or pigment dispersion sothat uniform incorporation of the IR radiation absorbing compoundthroughout the infrared radiation ablatable layer can be achieved.

The IR radiation absorbing compound(s), such as carbon blacks, arepresent in the infrared radiation ablatable layer generally in a totalamount of at least 1 weight % and up to and including 20 weight %, andtypically from about 2 to about 10 weight %, based on the total dryweight of the layer.

It is also possible that the infrared radiation absorbing compound (suchas a carbon black) is not merely dispersed uniformly within the infraredradiation ablatable layer, but it is present in a concentration that isgreater near the bottom surface than the image-forming surface. Thisconcentration profile can provide a laser energy absorption profile asthe depth into the infrared radiation ablatable layer increases. In someinstances, the concentration change is continuously and generallyuniformly increasing with depth. In other instances, the concentrationis varied with layer depth in a step-wise manner. Further details ofsuch arrangements of the IR radiation absorbing compound are provided incopending and commonly assigned U.S. Ser. No. 12/581,926 (filed Oct. 20,2009 by Landry-Coltrain, Burberry, Perchak, Ng, Tutt, Rowley, andFranklin).

Thus, some embodiments of the present invention infrared radiationablatable flexographic printing precursors comprise an infraredradiation ablatable layer comprising from about 1 to about 20 weight %(or from about 2 to about 10 weight %) of a conductive carbon blackhaving a dibutyl phthalate (DBP) adsorption of less than 110, and amixture a high molecular weight ethylene-propylene-diene terpolymer(EPDM) rubber and a low molecular weight EPDM rubber, wherein the weightratio of the high molecular weight EPDM to the low molecular weight EPDMrubber is from about 3:1 to about 5:1. Non-conductive carbon blacks arealso useful.

The infrared radiation ablatable layer can further comprise a carbonblack and one or more inorganic fillers. Useful inorganic fillersinclude but are not limited to, silica, calcium carbonate, magnesiumoxide, talc, barium sulfate, kaolin, bentonite, zinc oxide, mica, andtitanium dioxide, and mixtures thereof. Thus, useful inorganic fillerparticles are silica and alumina, such as fine particulate silica, fumedsilica, porous silica, surface treated silica, sold as Aerosil fromDegussa and Cab-O-Sil from Cabot Corporation, micropowders such asamorphous magnesium silicate cosmetic microspheres sold by Cabot and 3MCorporation, calcium carbonate and barium sulfate particles andmicroparticles. Particularly useful fillers are zinc oxide, calciumcarbonate, titanium dioxide, and silicas. The amount of inorganicfillers is generally at least 5 and up to and including 50 weight %,based on the total dry layer weight. However, more typically, the amountof inorganic fillers is from about 10 to about 35 weight %.

Contrary to the teaching in the prior art, for example, “Laser Engravingof Rubbers—The Influence of Fillers” by W. Kern et al., October 1997,710-715 (Rohstoffe Und Anwendendunghen) describing EPDM formulations wehave found that the use of inorganic fillers do not adversely affectsensitivity. This may be due to the presence of the lower molecularweight EPDM in the infrared radiation ablatable layer. Some fillers canalso improve the mechanical properties of the precursor.

If both a carbon black and inorganic filler are present, the weightratio of the carbon black to the inorganic filler(s) is from about 1:50to about 1:1.5, or from about 1:20 to about 1:5. We have found thatthese ratios are particularly useful in the preparation of flexographicprinting plate precursors even when the precursors have infraredradiation ablatable layers that are not based on EPDM elastomers.

It is also desirable that the infrared radiation ablatable layer furthercomprises a vulcanizer (or crosslinking agent) that can crosslink theEPDM rubbers and any other resins in the layer that can benefit fromcrosslinking Useful vulcanizers include but are not limited to, sulfurand sulfur-containing compounds, peroxide, hydroperoxides, and azocrosslinking agents. A mixture of sulfur and a peroxide can also beused, or a mixture of sulfur, a peroxide, and an azo crosslinking agentcan be used. The amount of vulcanizer that can be present in the layeris at least 0.5% and up to and including 5 weight %, based on the totaldry layer weight. Useful sulfur-containing compounds include but are notlimited to, zinc dibutyl dithiocarbamate (ZDBC), tetramethylthiuramdisulfide (TMTD), and tetramethylthiuram monosulfide (TMTM). Usefulperoxides include but are not limited to,di(t-butylperoxyisopropyl)benzene, 2,5-dimethyl-2,5bis(t-butyl)peroxy)hexane, dicumyl peroxide, and di(t-butyl)peroxide,and any others that can react with single carbon-carbon bonds and thusproduce a higher curing density and better compression set. Compressionset is important because it represents the resistance to changes inprinting by the printing plate being impacted on each printingimpression followed by a brief recovery between printing. Peroxidevulcanization gives less odor than sulfur vulcanization.

There are certain materials, however, that should be avoided in thepractice of this invention. Plasticizers such as mineral oils have beenfound to cause various problems. They tend to come to the surface duringgrinding and thus block the grinding medium. They also provide printingplates that may swell or lose material during long run printing and maysweat out during long term storage of the flexographic printingprecursors. This sweating out of the plasticizer can reduce the adhesionbetween the infrared radiation ablatable layer and a polyester substratecausing debonding or delamination either during printing or when theprinting plate is removed from the press after printing.

In some embodiments, microcapsules are dispersed within the infraredradiation ablatable layer. The “microcapsules” can also be known as“hollow beads”, “hollow spheres”, “microspheres”, microbubbles“,“micro-balloons”, “porous beads”, or “porous particles”. Such componentscan include a thermoplastic polymeric outer shell and either core of airor a volatile liquid such as isopentane and isobutane. Thesemicrocapsules include a single center core or many voids within thecore. The voids can be interconnected or non-connected. For example,non-laser-ablatable microcapsules can be designed like those describedin U.S. Pat. No. 4,060,032 (Evans) and U.S. Pat. No. 6,989,220 (Kanga)in which the shell is composed of apoly[vinylidene-(meth)acrylonitrile]resin or poly(vinylidene chloride),or as plastic micro-balloons as described for example in U.S. Pat. No.6,090,529 (Gelbart) and U.S. Pat. No. 6,159,659 (Gelbart).

The amount of microspheres that may be present is from about 2 to about70 weight % of the total dry layer weight. The microspheres can comprisea thermoplastic shell that is either hollow inside or enclosing ahydrocarbon or low boiling liquid. For example, the shell can becomposed of a copolymer of acrylonitrile and vinylidene chloride ormethacrylonitrile, methyl methacrylate, or a copolymer of vinylidenechloride, methacrylic acid, and acrylonitrile. If a hydrocarbon ispresent within the microspheres, it can be isobutene or isopentane.EXPANCEL® microspheres are commercially available from Akzo NobleIndustries (Duluth, Ga.). Dualite and Micropearl polymeric microspheresare commercially available from Pierce & Stevens Corporation (Buffalo,N.Y.). Hollow plastic pigments are available from Dow Chemical Company(Midland, Mich.) and Rohm and Haas (Philadelphia, Pa.). The microspheresgenerally have a diameter of 50 μm or less.

Inert microspheres can be hollow or filled with an inert solvent, andupon laser imaging, they burst and give a foam-like structure orfacilitate ablation of material from the infrared radiation ablatablelayer because they reduce the energy needed for ablation. Inertmicrospheres are generally formed of an inert polymeric or inorganicglass material such as a styrene or acrylate copolymer, silicon oxideglass, magnesium silicate glass, vinylidene chloride copolymers.

The amount of inert particulate materials or microspheres that can bepresent is from about 4 to about 70 weight % based on the total drylayer weight.

Optional addenda in the infrared radiation ablatable layer can includebut are not limited to, dyes, antioxidants, antiozonants, stabilizers,dispersing aids, surfactants, and adhesion promoters, as long as they donot interfere with ablation efficiency or vulcanization.

An infrared radiation ablatable flexographic printing precursorcomprises an infrared radiation ablatable layer comprising a carbonblack, one or more inorganic fillers, and one or more elastomers,wherein the weight ratio of the carbon black to the inorganic filler(s)is from about 1:50 to about 1:1.5 (or from about 1:20 to about 1:5). Theelastomers can include but are not limited to, copolymers of butadieneand styrene, copolymers of isoprene and styrene, styrene-diene-styreneblock copolymers such as polystyrene-polybutadiene-polystyrene,polystyrene-polyisoprene-polystyrene, andpolystyrene-poly(ethylenebutylene)-polystyrene. Elastomers also includenon-crosslinked polybutadiene and polyisoprene, nitrile elastomers,polychloroprene, polyisobutylene and other butyl elastomers,chlorosulfonated polyethylene, polysulfide, polyalkylene oxides,polyphosphazenes, elastomeric polymers and copolymers of acrylates andmethacrylates, elastomeric polyurethanes and polyesters, elastomericpolymers and copolymers of olefins such as ethylene-propylene copolymersand non-crosslinked EPDM, and elastomeric copolymers of vinyl acetateand its partially hydrogenated derivatives. In particular, theelastomers include a mixture of a high molecular weightethylene-propylene-diene terpolymer (EPDM) rubber and a low molecularweight EPDM rubber, as described above.

The flexographic printing precursors of this invention can be preparedin the following manner:

The mixture of a high molecular weight ethylene-propylene-dieneterpolymer (EPDM) rubber and a low molecular weight EPDM rubber isformulated with a desired weight ratio as described above. Additionalcomponents (such as inorganic fillers or a carbon black) can be addedand the resulting mixture is then compounded using standard equipmentfor rubber processing (for example, a 2-roll mill or internal mixer ofthe Banbury type). During the mixing, the temperature can rise to 90° C.due to the high shear forces in the mixing apparatus. This process takesfrom 5 to 30 minutes depending upon the batch size, amount of inorganicfillers, type of rubber resin, and other factors known to a skilledartisan. Alternatively, it is desirable to incorporate a small qualityof the inorganic filler(s) and a small quantity of the low molecularweight EPDM, until all of the ingredients are mixed with the highmolecular weight EPDM rubber. The compounded mixture with the tworubbers and any other components (such as antioxidants, inorganicfillers, vulcanizers, carbon black), in their un-vulcanized state, arefed into a calender where a continuous sheet of rubber is deposited ontoa carrier base (such as a fabric web) and wound into a continuous rollof infrared radiation ablatable layer on the fabric base.

Controlling the rubber sheet thickness is accomplished by adjusting thepressure between the calender rolls and the calendaring speed. In somecases, where the rubbery mixture does not stick to the calender rolls,the rolls are heated to improve the tackiness of the rubber and toprovide some adhesion to the calender rolls. This continuous roll ofcalendered material can be vulcanized in an autoclave under desiredtemperature and pressure conditions. For example, with a sulfurvulcanization system, the curing conditions are generally about 140° C.for up to 6 hours. Shorter times can be used if higher than atmosphericpressure is applied in the process. For peroxide curing systems, forexample with Parkadox 14/40 (Kayaku Akzo), the curing conditions wouldbe about 175° C. for up to 6 hours.

The continuous infrared radiation ablatable layer is then laminated to asuitable support, such as a polyester film. The continuous infraredradiation ablatable layer can be ground using suitable continuousgrinding apparatus to provide a uniform thickness in the continuous web.The web can then be cut to size to provide suitable flexographicprinting precursors.

The process for making flexographic printing sleeves is similar but thecompounded mixture is applied to a printing sleeve core to provide aninfrared radiation ablatable sleeve. This sleeve is then vulcanized in asuitable manner, and can be ground to a uniform thickness using suitableequipment.

The flexographic printing precursor can also be constructed with asuitable protective layer or slip film (with release properties or arelease agent) in a cover sheet that is removed prior to ablationimaging. Such a protective layer can be a polyester film [such aspoly(ethylene terephthalate)] forming the cover sheet. A backing layeron the substrate side opposite the infrared radiation ablatable layercan also be present.

Laser Ablation Imaging

Ablation energy is preferably applied using an IR radiation emittingdiode but carbon dioxide or YAG lasers can also be used. Ablation toprovide a relief image with a minimum depth of at least 50 μm is desiredwith a relief image having a minimum depth of at least 100 μm or atypical depth of from 300 to 1000 μm or up to 600 μm being desirable.The relief image may have a maximum depth up to about 100% of theoriginal thickness of the infrared radiation ablatable layer when asubstrate is present. In such instances, the floor of the relief imagemay be the substrate (if the ablatable layer is completely removed inthe imaged regions), a lower region of the infrared radiation ablatablelayer, or an underlayer such as an adhesive layer or compliant layer.When a substrate is absent, the relief image may have a maximum depth ofup to 80% of the original thickness of the original ablatable layer. AnIR diode laser operating at a wavelength of from about 700 to about 1400nm is generally used, and a diode laser operating at from 800 nm to 1250nm is useful for ablative imaging.

Generally, ablation imaging is achieved using at least one infraredradiation laser having a minimum fluence level of at least 20 J/cm² atthe element surface and typically infrared imaging is at from about 20to about 1000 J/cm² or typically from 50 to 800 J/cm².

A suitable laser engraver that would provide satisfactory ablation isdescribed in WO 2007/149208 (Eyal et al.) that is incorporated herein byreference. This laser engraver is considered to be a “high power” laserablating imager or engraver and has at least two laser diodes emittingradiation in one or more wavelengths so that imaging with the one ormore wavelengths is carried out at different depths relative to theprecursor surface. For example, the multi-beam optical head described inthis publication incorporates numerous laser diodes each having a powerin the order of at least 10 Watts per emitter width of 100 μm. Theselasers can be modulated directly at relatively high frequencies withoutthe need for external modulators.

Thus, ablative imaging can be carried out at the same or differentdepths relative to the surface of the infrared radiation ablatable layerusing two or more laser diodes each emitting radiation in one or morewavelengths.

Other imaging (or engraving) devices and components thereof and methodsare described for example in U.S. Patent Application Publications2008/0153038 (Siman-Tov et al.) describing a hybrid optical head fordirect engraving, 2008/0305436 (Shishkin) describing a method of imagingone or more graphical pieces in a flexographic printing plate precursoron a drum, 2009/0057268 (Aviel) describing imaging devices with at leasttwo laser sources and mirrors or prisms put in front of the lasersources to alter the optical laser paths, and 2009/0101034 (Aviel)describing an apparatus for providing an uniform imaging surface, all ofwhich publications are incorporated herein by reference. In addition,copending and commonly assigned U.S. Ser. No. 12/502,267 (filed Jul. 14,2009 by Matzner, Aviel, and Melamed) describes an engraving systemincluding an optical imaging head, a printing plate construction, and asource of imaging radiation, which copending application is incorporatedherein by reference. Copending and commonly assigned U.S. Ser. No.12/555,003 (filed Sep. 8, 2009 but Aviel and Eyal) describes an imaginghead for 3D imaging of flexographic printing plate precursors usingmultiple lasers, which copending application is also incorporated hereinby reference.

Thus, a system for providing flexographic printing plates or sleevesinclude one or more of the flexographic printing precursors describedabove, as well as one or more groups of one or more sources of imaginginfrared radiation, each source capable of emitting infrared radiation(see references cited above). Such imaging sources can include but arenot limited to, laser diodes, multi-emitter laser diodes, laser bars,laser stacks, fiber lasers, or a combination thereof. The system canalso include one or more sets of optical elements coupled to the sourcesof imaging infrared radiation to direct imaging infrared radiation fromthe sources onto the flexographic printing precursor (see referencescited above for examples of optical elements).

Ablation to form a relief image can occur in various contexts. Forexample, sheet-like elements can be imaged and used as desired, orwrapped around a printing sleeve core or cylinder form before imaging.The flexographic printing precursor can also be a printing sleeve thatcan be imaged.

During imaging, products of ablation can be gaseous or volatile andreadily collected by vacuum for disposal or chemical treatment. Anysolid debris from ablation can be collected and removed using suitablemeans such as vacuum, compressed air, brushing with brushes, rinsingwith water, ultrasound, or any combination of these.

During printing, the resulting flexographic printing plate or printingsleeve is inked using known methods and the ink is appropriatelytransferred to a suitable substrate such as paper, plastics, fabrics,paperboard, or cardboard.

After printing, the flexographic printing plate or sleeve can be cleanedand reused and a printing cylinder can be scraped or otherwise cleanedand reused as needed. Cleaning can be accomplished with compressed air,water, or a suitable aqueous solution, or by rubbing with cleaningbrushes or pads.

The present invention provides at least the following embodiments andcombinations thereof:

1. An infrared radiation ablatable flexographic printing precursor thatcomprises an infrared radiation ablatable layer comprising a mixture ofa high molecular weight ethylene-propylene-diene terpolymer (EPDM)rubber and a low molecular weight EPDM rubber.

2. The precursor of embodiment 1 wherein the weight ratio of the highmolecular weight EPDM to the low molecular weight EPDM rubber is fromabout 2:1 to about 10:1.

3. The precursor of embodiment 1 or 2 wherein the weight ratio of thehigh molecular weight EPDM to the low molecular weight EPDM rubber isfrom about 3:1 to about 5:1.

4. The precursor of any of embodiments 1 to 3 wherein the molecularweight of the high molecular weight EPDM is from about 200,000 to about800,000, and the molecular weight of the low molecular weight EPDM isfrom about 2,000 to about 10,000.

5. The precursor of any of embodiments 1 to 4 wherein the molecularweight of the high molecular weight EPDM is from about 250,000 to about500,000, and the molecular weight of the low molecular weight EPDM isfrom about 2,000 to about 8,000.

6. The precursor of any of embodiments 1 to 5 wherein the infraredradiation ablatable layer further comprises a carbon black.

7. The precursor of any of embodiments 1 to 6 wherein the infraredradiation ablatable layer further comprises a conductive carbon black.

8. The precursor of any of embodiments 1 to 7 wherein the infraredradiation ablatable layer further comprises a conductive carbon blackhaving a dibutyl phthalate (DBP) absorption of less than 110.

9. The precursor of embodiments 1 to 6 wherein the infrared radiationablatable layer further comprises a non-conductive carbon black.

10. An infrared radiation ablatable flexographic printing precursorcomprises an infrared radiation ablatable layer comprising from about 1to about 20 weight % of a conductive carbon black having a dibutylphthalate (DBP) adsorption of less than 110, and a mixture of a highmolecular weight ethylene-propylene-diene terpolymer (EPDM) rubber and alow molecular weight EPDM rubber, wherein the weight ratio of the highmolecular weight EPDM to the low molecular weight EPDM rubber is fromabout 3:1 to about 5:1.

11. The precursor of embodiment 10 wherein the infrared radiationablatable layer comprises from about 2 to about 10 weight % of theconductive carbon black.

12. The precursor of any of embodiments 1 to 11 wherein the infraredradiation ablatable layer further comprises a vulcanizer

13. The precursor of any of embodiment 12 wherein the infrared radiationablatable layer further comprises sulfur or a peroxide as a vulcanizerand an azo crosslinking agent, or a mixture of sulfur and a peroxide, ora mixture of sulfur, an azo crosslinking agent, and a peroxide.

14. The precursor of any of embodiments 1 to 13 further comprising apolyester support upon which the infrared radiation ablatable layer isdisposed.

15. The precursor of any of embodiments 1 to 14 further comprising afabric support upon which the infrared radiation ablatable layer isdisposed.

16. The precursor of embodiment 15 wherein the fabric support isdisposed on a polyester support.

17. An infrared radiation ablatable flexographic printing precursorcomprises an infrared radiation ablatable layer comprising one or moreinorganic fillers, a carbon black, and a mixture of a high molecularweight ethylene-propylene-diene terpolymer (EPDM) rubber and a lowmolecular weight EPDM rubber, wherein the weight ratio of the highmolecular weight EPDM to the low molecular weight EPDM rubber is fromabout 2:1 to about 10:1.

18. The precursor of any of embodiments 1 to 17 wherein the infraredradiation ablatable layer further comprises one or more inorganicfillers that are chosen from silica, calcium carbonate, barium sulfate,kaolin, bentonite, zinc oxide, mica, and titanium dioxide.

19. An infrared radiation ablatable flexographic printing precursorcomprises an infrared radiation ablatable layer comprising:

from about 10 to about 35 weight % of one or more inorganic fillers andfrom about 1 to about 20 weight % of a carbon black, wherein the weightratio of the carbon black to the inorganic filler(s) is from about 1:50to about 1:1.5, and a mixture of a high molecular weightethylene-propylene-diene terpolymer (EPDM) rubber and a low molecularweight EPDM rubber, wherein the weight ratio of the high molecularweight EPDM to the low molecular weight EPDM rubber is from about 2:1 toabout 10:1.

20. A method of preparing the flexographic printing plate precursor ofany of embodiments 1 to 19 comprising:

A) providing a mixture of a high molecular weightethylene-propylene-diene terpolymer (EPDM) rubber and a low molecularweight EPDM rubber,

B) adding optional components, and compounding the resulting mixture ina two-roll mill,

C) applying the compounded mixture to a fabric base to provide acontinuous roll of an infrared radiation ablatable layer,

D) causing vulcanization in the continuous infrared radiation ablatablelayer,

E) laminating a polyester support to the continuous infrared radiationablatable layer to provide a continuous laminated web, and

F) grounding the infrared radiation ablatable layer.

21. The method of embodiment 20 wherein the infrared radiation ablatablelayer is ground in the continuous laminated web to a uniform thickness.

22. The method of embodiment 20 or 21 wherein the mixture of highmolecular with EPDM and low molecular weight EPDM further comprises acarbon black in an amount of from about 1 to about 20 weight %, and theweight ratio of the high molecular weight EPDM rubber to the lowmolecular weight EPDM rubber is from about 2:1 to about 10:1.

23. The method of any of embodiments 20 to 22 wherein the mixture ofhigh molecular with EPDM and low molecular weight EPDM further comprisesone or more inorganic fillers, a vulcanizer, or both an inorganic fillerand a vulcanizer.

24. The method of any of embodiments 20 to 23 wherein the continuouslaminated web further comprises a fabric layer between the polyestersupport and the continuous infrared radiation ablatable layer.

25. A method of preparing the flexographic printing sleeve precursor ofany of embodiments 1 to 19 comprising:

A) providing a mixture of a high molecular weightethylene-propylene-diene terpolymer (EPDM) rubber and a low molecularweight EPDM rubber,

B) adding optional components, and compounding the resulting mixture ina two-roll mill,

C) applying the compounded mixture to a printing sleeve core to providean infrared radiation ablatable sleeve,

D) causing vulcanization in the infrared radiation ablatable sleeve, and

E) smoothing the continuous infrared radiation ablatable sleeve to auniform thickness.

26. The method of embodiment 25 wherein the mixture of high molecularweight EPDM and low molecular weight EPDM further comprises a carbonblack, and optionally one or more inorganic fillers and a vulcanizer

27. A method of providing flexographic printing plate or sleevecomprising:

imaging the flexographic printing precursor of any of embodiments 1 to19 using infrared radiation to provide a relief image in the infraredradiation ablatable layer.

28. The method of embodiment 27 wherein imaging is carried out using alaser at a power of at least 20 J/cm².

29. The method of embodiment 27 or 28 further comprising removal ofdebris after imaging.

30. The method of embodiment 29 wherein debris is removed by vacuum,compressed air, brushes, rinsing with water, ultrasound, or anycombination of these.

31. The method of any of embodiments 27 to 30 wherein imaging is carriedout using a high power laser ablating imager.

32. The method of any of embodiments 27 to 31 wherein imaging is carriedout at the same or different depths relative to the surface of theinfrared radiation ablatable layer using two or more laser diodes eachemitting radiation in one or more wavelengths.

33. A system for providing a flexographic printing plate or printingsleeve, comprising:

the flexographic printing precursor of any of embodiments 1 to 19,

a group of one or more sources of imaging infrared radiation, eachsource capable of emitting infrared radiation,

a set of optical elements coupled to the sources of imaging infraredradiation to direct imaging infrared radiation from the sources onto theflexographic printing precursor. 34. The system of embodiment 33 whereinthe sources of imaging infrared radiation are laser diodes,multi-emitter laser diodes, laser bars, laser stacks, fiber lasers, or acombination thereof.

35. An infrared radiation ablatable flexographic printing precursorcomprises an infrared radiation ablatable layer comprising a carbonblack, one or more inorganic fillers, and one or more elastomers,wherein the weight ratio of the carbon black to the inorganic filler(s)is from about 1:50 to about 1:1.5.

36. The precursor of embodiment 35 wherein the elastomers includes amixture of a high molecular weight ethylene-propylene-diene terpolymer(EPDM) rubber and a low molecular weight EPDM rubber.

37. The precursor of embodiment 35 or 36 wherein the weight ratio of thecarbon black to the inorganic filler(s) is from about 1:20 to about 1:5

The following Examples are provided to illustrate the present inventionand are not to be limiting in any manner.

EXAMPLES

Comparisons of commercially available laser-ablatable flexographicprinting plate precursors were made to flexographic printing plateprecursors of the present invention as described below. The comparisonmethod was to measure the depth of engraving under standard conditionsusing a laser diode engraver with a constant drum angular velocity of100 rpm and constant laser power of 4.5 watts. Typical engraving depthsfor the commercial printing plate precursors were compared to theprecursors made according to Invention Example 1 and the data are asfollows:

Printing Plate Precursor Engraving depth (μm) Invention Example 1 64Comparative Example 1 (Bottcher 47 SBR) Comparative Example 2 (Printec)45

Surface Evenness:

The demands on achieving high surface evenness have increased because ofthe need to accurately reproduce delicate half tones. Thus, forinstance, U.S. Pat. No. 5,798,202 (noted above) specifically teachesaway from the use of vulcanized rubbers because the resultingflexographic printing plate precursors must be ground down to a uniformsurface. Grinding is time-consuming. However, we have found that it isnot possible to reach the high level of surface evenness withoutgrinding and have found a method of doing so in a continuous manner tocontinuous forms, thus minimizing the time necessary for grinding andthereby making it commercially viable.

In addition to the production of even surfaces, investigation has alsodetermined that the unground surface of the flexographic precursor tendsto be smooth and shiny due to the formation of a continuous surface filmover any pigment in the outer layer. Grinding has the effect ofdisrupting this film so that the oleophilic pigment such as carbonblack, which is present in more completely exposed regions, impartsimproved ink accepting properties to the flexographic precursor. Inaddition, grinding produces a very accurate flexographic precursorthickness that improves printing performance. We found it necessary toprovide laser-engravable formulations that will have optimized printingproperties when the printing surface has been ground. Also, we foundthat successful grinding is dependent upon the laser-engravableformulation. For instance, the presence of plasticizers, such as processoils (mineral oils) that are regarded in the rubber industry asessential for calendering to produce standard sheets of rubber, causessevere problems if the sheets must be ground, as is the case for thehigh quality flexographic precursors of the present invention. Duringgrinding, flexographic precursors containing mineral oils produce asticky mixture of oil and ground rubber that clogs the medium used forgrinding, making continuous grinding impossible.

Print Quality:

While print quality can be qualitatively assessed overall andquantitatively measured by certain parameters, it is difficult topredict how a given laser-engravable formulation may affect this.Optical density of solid print areas is an important parameter.Sharpness is partially defined by distinguishable line width and thecomparative examination of 2-point text under a low-powered microscope.

Means of Flexographic Printing Precursor Production:

Top quality printing requires high quality flexographic plate and sleeveproduction, and thus methods of production described in the art are notsufficiently reliable for use in the present invention. For example,production by laying down multiple layers from solvents would be tooprone to the incidence of solvent bubbles during the drying process inwhich the solvent is removed from the coating. Whereas vulcanization isa proven method that may involve extrusion and calendering without thedanger of clogging during compounding, catalysis of reactions such asurethane formation during compounding and possible extrusion can causeblocking of the production equipment. U.S. Pat. No. 5,796,202 describesa variety of methods for flexographic printing precursor production butdoes not appreciate that the combination of extrusion, calendering, andgrinding is the only method that will produce sufficient flexographicprecursor quality.

In some prior art, processing oils are used in laser-ablatableflexographic printing plates. However, such formulations cause problemsduring continuous grinding procedures. On the other hand, processingoils are needed for the production of EPDM sheets. Without such oils,EPDM is too dry and hard and does not adhere sufficiently to thecalendar rollers for proper calendering. The processing oils promotegood calendering by improving tack and by lowering the viscosity.However, the processing oils introduce other problems and inhibit longprint run capability. The present invention solves this problem ofcalendering, grinding, and run length by including a low molecularweight EPDM in the laser-engravable formulation.

Long Run Length Capability:

The use of flexographic printing in the packaging industry frequentlyrequires long printing runs for consumer products such as packaging offood-stuffs. Many known laser-engravable flexographic printing plateprecursors include plasticizers. The printing process utilizes inksbased on solvent mixtures or water. During long printing runs,plasticizers may be extracted by the inks solvents and the ink solventsabsorbed. Due to these effects, flexographic printing precursors willchange in durometer hardness and may either swell or shrink. Any changeswill affect the quality of prints, making it variable. Consequently, thechoice of materials in the laser-engravable layer (such as theelastomer) is important. Historically, this importance is not been fullyappreciated in the art and the inclusion of plasticizers show a lack ofappreciation for the highest standards of retention of integrity of theformulation during the print run. We found that such plasticizers leachout during long print runs as well as during the process of grindingwhen the precursors are being prepared.

Invention Example 1

One hundred parts by weight of an EPDM rubber was masticated in a tworoller mill. The grade of EPDM as based on ethylidene norborene and wasthe commercial grade KEP240 (sold by Kumho). Mastication was continueduntil the shapeless lump placed in the mill had been formed into asemi-transparent sheet. This sheet was rolled up and fed into a Banburymixer operating at between 70 and 80° C. During this mixing, thefollowing components (parts by weight) were added individually in theorder shown below.

Stearic Acid 1.0 part Zinc oxide 6.25 parts Carbon black 12.0 parts

The following two ingredients were then added, approximately one thirdat a time, firstly one third of the silica, one third of the liquid EPDMrubber, and then the next third of the silica and so on until thequantities had been completely added

Silica 30 parts Trilene 67 EPDM 20 partsTrilene 67 is a liquid EPDM rubber sold by Lion Polymers and has amolecular weight of approximately 7700 and a Brookfield viscosity at100° C. of 128,000.

The entire mixture was mixed for approximately 20 minutes in the Banburymixer until a constant stress reading could be observed on the Banburymixer. The resulting material was removed from the Banbury mixer as ahomogenous lump that was fed onto a two roller mill and the followingmaterials added were then added:

6 parts by weight of a silane coupling agent,bis[3-(triethoxysilyl)propyl]polysulfide,

10 parts by weight of di-(t-butylperoxyisopropyl)benzene, and

1.5 parts of 2,4,6-triallyoxy-1,3,5-triazine co-agent.

The Mooney viscosity of the resulting mixture was about 53. Mooneyviscosities need to be between 30 and 80 or more preferably, between 40and 60. Higher and lower viscosities than these values will not allowprocessability on a two roller mill.

The milled material was then fed through a calendar at a temperature of30-80° C. together with a fabric base. The calendar gap was pre-set tothe thickness requirements. The resulting roll of laminated rubber andfabric was fed into an autoclave at 135° C. for a period of time. Aftercooling the roll to room temperature, it was laminated to a 125 μmpoly(ethylene terephthalate) film and post cured in an autoclave at 120°C.

The completed flexographic printing plate precursor was continuouslyground on the non-polyester side to a uniform thickness by a buffingmachine.

The precursor was cut to an appropriate size and placed on a laserablating plate imager where excellent sharp deep relief images wereproduced that were used on a flexographic printing press to producehundreds of thousands of sharp, clean impressions. The compression setfor this printing plate precursor was measured according to ASTM D 395Method B and found to be 13%.

Comparative Example 1

Invention Example 1 was repeated but a mineral oil was substituted forthe liquid EPDM rubber. Although the formulation was processable to formflexographic printing precursor sheets, they could not be ground to givegood evenness in the laser-ablatable surface.

In addition, when the sheets were soaked in a solvent mixture of ethylacetate (20%) and isopropanol (80%) for 24 hours, we determined that themineral oil leached out with a weight loss of 1.5% compared to 0% lossfrom the flexographic printing plate precursors of Invention Example 1.Measurements of swelling showed that the Invention Example 1flexographic printing precursor swelled by 3.2% while the ComparativeExample 1 precursor swelled by 4.6%.

Comparative printing tests showed that the Invention Example 1flexographic printing plates provided better ink transfer, less dot gainthat can be seen in smaller sharper 50% dots, thinner 50 μm lines, andmore constant print quality for the 100^(th) impression as compared tothe 10,000^(th) impression.

The Invention Example 1 precursor had a Taber abrasion weight loss percycle of 0.350 mgr compared to the Comparative Example 1 precursor valueof 0.447 mgr. The compression set for this printing plate precursor wasmeasured according to ASTM D 395 Method B and found to be 19%.

Comparative Example 2

Invention Example 1 was repeated, but SBR 1502 rubber was substitutedfor the EPDM rubber mixture. The SBR 1502 rubber required no addition ofmineral oil to make it manufacturable but it exhibited a swelling of9.9% using the test described in Comparative Example 1.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

1. An infrared radiation ablatable flexographic printing precursor thatcomprises an infrared radiation ablatable layer comprising a mixture ofa high molecular weight ethylene-propylene-diene terpolymer (EPDM)rubber and a low molecular weight EPDM rubber.
 2. The precursor of claim1 wherein the weight ratio of the high molecular weight EPDM to the lowmolecular weight EPDM rubber is from about 2:1 to about 10:1.
 3. Theprecursor of claim 1 wherein the weight ratio of the high molecularweight EPDM to the low molecular weight EPDM rubber is from about 3:1 toabout 5:1.
 4. The precursor of claim 1 wherein the molecular weight ofthe high molecular weight EPDM is from about 200,000 to about 800,000,and the molecular weight of the low molecular weight EPDM is from about2,000 to about 10,000.
 5. The precursor of claim 1 wherein the molecularweight of the high molecular weight EPDM is from about 250,000 to about500,000, and the molecular weight of the low molecular weight EPDM isfrom about 2,000 to about 8,000.
 6. The precursor of claim 1 wherein theinfrared radiation ablatable layer further comprises a carbon black. 7.The precursor of claim 1 wherein the infrared radiation ablatable layerfurther comprises a conductive carbon black.
 8. The precursor of claim 1wherein the infrared radiation ablatable layer further comprises aconductive carbon black having a dibutyl phthalate (DBP) absorption ofless than
 110. 9. The precursor of claim 1 wherein the infraredradiation ablatable layer further comprises a non-conductive carbonblack.
 10. An infrared radiation ablatable flexographic printingprecursor comprises an infrared radiation ablatable layer comprisingfrom about 1 to about 20 weight % of a conductive carbon black having adibutyl phthalate (DBP) adsorption of less than 110, and a mixture of ahigh molecular weight ethylene-propylene-diene terpolymer (EPDM) rubberand a low molecular weight EPDM rubber, wherein the weight ratio of thehigh molecular weight EPDM to the low molecular weight EPDM rubber isfrom about 3:1 to about 5:1.
 11. The precursor of claim 10 wherein theinfrared radiation ablatable layer comprises from about 2 to about 10weight % of the conductive carbon black.
 12. The precursor of claim 1wherein the infrared radiation ablatable layer further comprises avulcanizer.
 13. The precursor of claim 12 wherein the infrared radiationablatable layer further comprises sulfur or a peroxide as a vulcanizerand an azo crosslinking agent, or a mixture of sulfur and a peroxide, ora mixture of sulfur, an azo crosslinking agent, and a peroxide.
 14. Theprecursor of claim 1 further comprising a polyester support upon whichthe infrared radiation ablatable layer is disposed.
 15. The precursor ofclaim 1 further comprising a fabric support upon which the infraredradiation ablatable layer is disposed.
 16. The precursor of claim 15wherein the fabric support is disposed on a polyester support.
 17. Aninfrared radiation ablatable flexographic printing precursor comprisesan infrared radiation ablatable layer comprising one or more inorganicfillers, a carbon black, and a mixture of a high molecular weightethylene-propylene-diene terpolymer (EPDM) rubber and a low molecularweight EPDM rubber, wherein the weight ratio of the high molecularweight EPDM to the low molecular weight EPDM rubber is from about 2:1 toabout 10:1.
 18. The precursor of claim 17 wherein the infrared radiationablatable layer further comprises one or more inorganic fillers that arechosen from silica, calcium carbonate, barium sulfate, kaolin,bentonite, zinc oxide, mica, and titanium dioxide.
 19. An infraredradiation ablatable flexographic printing precursor comprises aninfrared radiation ablatable layer comprising: from about 10 to about 35weight % of one or more inorganic fillers and from about 1 to about 20weight % of a carbon black, wherein the weight ratio of the carbon blackto the inorganic filler(s) is from about 1:50 to about 1:1.5, and amixture of a high molecular weight ethylene-propylene-diene terpolymer(EPDM) rubber and a low molecular weight EPDM rubber, wherein the weightratio of the high molecular weight EPDM to the low molecular weight EPDMrubber is from about 2:1 to about 10:1. 20.-26. (canceled)
 27. A methodof providing flexographic printing plate or sleeve comprising: imagingthe flexographic printing precursor of claim 1 using infrared radiationto provide a relief image in the infrared radiation ablatable layer. 28.The method of claim 27 wherein imaging is carried out using a laser at apower of at least 20 J/cm².
 29. The method of claim 27 furthercomprising removal of debris after imaging.
 30. The method of claim 29wherein debris is removed by vacuum, compressed air, brushes, rinsingwith water, ultrasound, or any combination of these.
 31. The method ofclaim 27 wherein imaging is carried out using a high power laserablating imager.
 32. The method of claim 27 wherein imaging is carriedout at the same or different depths relative to the surface of theinfrared radiation ablatable layer using two or more laser diodes eachemitting radiation in one or more wavelengths.
 33. A system forproviding a flexographic printing plate or printing sleeve, comprising:the flexographic printing precursor of claim 1, a group of one or moresources of imaging infrared radiation, each source capable of emittinginfrared radiation, a set of optical elements coupled to the sources ofimaging infrared radiation to direct imaging infrared radiation from thesources onto the flexographic printing precursor.
 34. The system ofclaim 33 wherein the sources of imaging infrared radiation are laserdiodes, multi-emitter laser diodes, laser bars, laser stacks, fiberlasers, or a combination thereof.
 35. An infrared radiation ablatableflexographic printing precursor comprises an infrared radiationablatable layer comprising a carbon black, one or more inorganicfillers, and one or more elastomers, wherein the weight ratio of thecarbon black to the inorganic filler(s) is from about 1:50 to about1:1.5.
 36. The precursor of claim 35 wherein the elastomers includes amixture of a high molecular weight ethylene-propylene-diene terpolymer(EPDM) rubber and a low molecular weight EPDM rubber.
 37. The precursorof claim 35 wherein the weight ratio of the carbon black to theinorganic filler(s) is from about 1:20 to about 1:5